Method for producing water-absorbent resins

ABSTRACT

In a process for preparing water-absorbent resins based on acrylic acid, crude acrylic acid is firstly isolated from the reaction gases from the catalytic gas-phase oxidation of propane, propylene and/or acrolein. This is treated with an aldehyde scavenger and pure acrylic acid is separated by distillation from the treated crude acrylic acid, and this pure acrylic acid can be subjected directly to a free-radical polymerization.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Water-absorbent resins based on acrylic acid, in particular those knownas superabsorbents, are widely used in, for example, hygiene articles.

2. Description of the Background

It is known that acrylic acid can be prepared by heterogeneouslycatalyzed gas-phase oxidation of propane, propylenene (and/or acrolein)by means of molecular oxygen over solid catalysts. The gas-phaseoxidation of propane, propylene and/or acrolein forms the desiredacrylic acid together with by-products, in particular in the form ofcarbonyl compounds, e.g. aldehydes such as benzaldehyde, furfurals,propionaldehyde etc., and also formic acid and maleic acid or maleicanhydride, whose presence has an adverse effect on the polymerization ifthe acrylic acid is used for preparing superabsorbents. Aldehydes hinderthe polymerization of the acrylic acid and also lead to discoloredpolymers. The presence of maleic anhydride in the polymerization leadsto formation of undesired copolymers which influence the properties ofthe desired polymers. Contamination by carboxylic acids which are notcapable of polymerization, e.g. formic acid, are particularlydisadvantageous when the polyacrylic acids prepared by polymerizationare used in hygiene articles which come into contact with human skin,since these carboxylic acids cause extreme skin irritation. The presenceof oligomeric acrylic acids, e.g. diacrylic, triacryilic or tetraacrylicacid, is troublesome because the oligomeric acrylic acid presentdissociates during the thermal treatment of the polyacrylic acid andliberates acrylic acid, which likewise causes extreme skin irritation.

In the preparation and/or isolation of acrylic acid, use is generallymade of process polymerization inhibitors such as phenothiazine,hydroquinone or hydroquinone monomethyl ether to suppress undesirablepremature polymerization, particularly at elevated temperatures.Undesired polymer formation leads to deposits on heat exchanger surfacesand column trays and also to blockage of lines, pumps, valves etc. Sincethe process polymerization inhibitors, which have an excellentinhibiting action, naturally also retard intended preparation ofpolyacrylic acid, they subsequently have to be removed again from theacrylic acid obtained and be replaced by storage polymerizationinhibitors which have less of an inhibiting action.

To remove or reduce the amounts of the abovementioned by-products andimpurities, multistage distillation and/or extraction and/orcrystallization steps are generally employed in the prior art. Acrylicacid which is obtainable in this way and is suitable for preparingabsorbent resins is generally referred to as pure acrylic acid. Thus, EP0 754 671 discloses a distillation process for purifying acrylic acidprepared by catalytic oxidation of propylene, in particular to removemaleic anhydride. EP 0 727 408 discloses a two-stage distillation ofcrude acrylic acid. The fraction taken off at the top, which comprisesformic acid and acetic acid, is esterified to make use of the aceticacid.

DE 2 241 714 discloses a process for separating acrylic acid from thereaction gases from the oxidation of propylene or acrolein by means ofcountercurrent absorption, with acetic acid and some water beingstripped from the resulting absorption solution by means of inert gases.DE 4 308 087 describes a process for separating acrylic acid from thereaction gases from the catalytic oxidation of propylene and/or acroleinby countercurrent absorption using a mixture of diphenyl ether, biphenyland dimethyl o-phthalate.

DE 196 34 614 discloses a process for separating off pure (meth)acrylicacid by distillation in a distillation apparatus which comprises a thinfilm evaporator, a condenser and a connection comprising an impingementdevice between thin film evaporator and condenser.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forpreparing water-absorbent resins, in which acrylic acid is separated ina technically simple manner from the reaction gases from the catalyticgas-phase oxidation of propane, propylene and/or acrolein and whichgives an acrylic acid which can be used directly without furtherpurification for preparing the water-absorbent resins.

We have found that this object is achieved by a process for preparingwater-absorbent resins, which comprises

-   a) obtaining crude acrylic acid by either    -   a1) absorbing acrylic acid from the reaction gases from the        catalytic gas-phase oxidation of propane, propylene and/or        acrolein in an absorption liquid and isolating crude acrylic        acid from the absorption liquid laden with acrylic acid,    -   or    -   a2) separating a crude acrylic acid fraction from the reaction        gases by fractional condensation and, if appropriate, subjecting        the fraction to purification by crystallization,-   b) treating the crude acrylic acid with an aldehyde scavenger,-   c) separating pure acrylic acid from the treated crude acrylic acid    by distillation, and-   d) subjecting the pure acrylic acid, if appropriate after partial    neutralization, if desired in admixture with further ethylenically    unsaturated monomers, to a free-radical polymerization.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The pure acrylic acid obtained in step c) generally contains less than 1ppm of process polymerization inhibitor such as phenothiazine, less than5 ppm of formic acid, less than 5 ppm of aldehyde and less than 100 ppmof diacrylic acid.

The oxidation of propane, propylene and/or acrolein to form acrylic acidin the gas phase is carried out in a manner known per se. The feed,which may have been mixed with an inert diluent gas, is passed inadmixture with oxygen over at least one heterogeneous catalyst,generally a mixed oxide catalyst comprising transition metals, e.g.molybdenum, vanadium, tungsten and/or iron, at elevated temperatures,usually from 200 to 400° C., and at atmospheric or superatmosphericpressure and is thus converted into acrylic acid by oxidation. Thereaction can be carried out in one or two stages. In a two-stagereaction, the propylene is oxidized to acrolein in a first stage and theacrolein is oxidized to acrylic acid in a second stage. Heterogeneouscatalysts used in the first stage are preferably oxidic multicomponentcatalysts based on the oxides of molybdenum, bismuth and iron, whilepreferred catalysts in the second stage are appropriate catalysts basedon the oxides of molybdenum and vanadium.

The conversion of propane, propylene and/or acrolein into acrylic acidis strongly exothermic. The feed stream is therefore advantageouslydiluted with an inert diluent gas, e.g. atmospheric nitrogen, carbondioxide, methane and/or steam. Although the type of reactors used is notsubject to any particular restriction, use is advantageously made ofshell-and-tube heat exchangers charged with the oxidation catalyst(s),since in these the major part of the heat liberated in the reaction canbe removed by convection and radiation at the cooled walls of the tubes.The reaction gases obtained in the single—or two-stage catalyticgas-phase oxidation usually comprises acrylic acid together withunreacted propane, propylene and/or acrolein, steam, carbon monoxide,carbon dioxide, nitrogen, oxygen, acetic acid, propionic acid,formaldehyde, further aldehydes and maleic acid or maleic anhydride. Thereaction gases typically comprise:

acrylic acid from 1 to 30% by weight propylene from 0.05 to 1% by weightacrolein from 0.01 to 2% by weight propane from 0.01 to 2% by weightsteam from 1 to 30% by weight carbon oxides from 0.05 to 15% by weightnitrogen from 0 to 90% by weight oxygen from 0.05 to 10% by weightformic acid from 0.01 to 1% by weight acetic acid from 0.05 to 2% byweight propionic acid from 0.01 to 2% by weight aldehydes from 0.01 to3% by weight maleic anhydride from 0.01 to 0.5% by weight

Crude acrylic acid having an acrylic acid content of usually at least99% by weight is firstly isolated from the reaction gases. Methods ofisolating crude acrylic acid from the reaction gases are known per se.In one embodiment of the process of the present invention, acrylic acidis absorbed from the reaction gases by means of an absorption liquid.Suitable absorption liquids are liquids in which acrylic acid has apronounced solubility, e.g. liquids which have a boiling point higherthan that of acrylic acid (hereinafter, “high-boiling liquid”) and whoseboiling point is preferably above 160° C. (at 1 atm). Possiblehigh-boiling liquids are, for example, biphenyl, diphenyl ether,dimethyl phthalate, ethylhexanoic acid, N-methylpyrrolidone, paraffinfractions and mixtures thereof. Alternatively, oligomeric acrylic acids,e.g. mixtures comprising diacrylic, triacrylic and tetraacrylic acids,can be used as high-boiling liquid. Biphenyl, diphenyl ether, dimethylo-phthalate and mixtures thereof are preferred, in particular mixturescomprising from 25 to 30% by weight of biphenyl and from 70 to 75% byweight of diphenyl ether together with, based on the mixture, from 0.1to 25% by weight of dimethyl o-phthalate.

Water is also a suitable absorption liquid.

The absorption liquid is brought into intimate contact with the reactiongases in an appropriate manner. For this purpose, the reaction gases areadvantageously passed through an absorption column in countercurrentflow to the descending absorption liquid. Examples of absorption columnswhich can be used are columns containing random or ordered packing,valve tray columns of bubble cap tray columns.

The reaction gases, which are generally at a temperature of from 200 to400° C., are preferably cooled to a suitable absorption temperature of,for example, from 100 to 180° C. before being introduced into theabsorption column. Cooling of the reaction gases to the absorptiontemperature can be carried out by indirect cooling, e.g. by means of aheat exchanger. However, this cooling is preferably carried out bydirect contact with a cooling liquid, preferably in a spray scrubber.The cooling liquid is advantageously largely separated off again in aseparator, cooled and recirculated before the reaction gases enter theabsorption column. The cooling liquid is preferably identical to theliquid which is used for the subsequent absorption of the acrylic acidfrom the reaction gases.

The absorption liquid laden with acrylic acid generally comprises notonly acrylic acid but also volatile impurities such as water, acrolein,formaldehyde and also formic acid and acetic acid. Secondary componentssuch as water, acrolein, formaldehyde and the aceteic and formic acidscan be at least partly removed by stripping with a stripping gas,particularly when using a high-boiling liquid as absorption liquid. Forthis purpose, the absorption liquid laden with acrylic acid is passedthrough a desorption column in countercurrent to a stripping gas, e.g.nitrogen or air. The amount of stripping gas required depends, inparticular, on the desorption temperature, which is advantageously20–50° C. higher than the absorption temperature; the stripping step ispreferably carried out at the same pressure as the absorption step. Theamount of stripping gas is preferably, based on the amount of reactiongas, from 5 to 25% by volume. The desorption column can be, for example,a column containing random or ordered packing, a valve tray column or abubble cap tray column.

The cooling liquid and/or the absorption liquid usually contain anamount of, for example, from 0.01 to 1% by weight of at least oneprocess polymerization inhibitor such as phenothiazine, phenoliccompounds such as hydroquinone, hydroquinone monomethyl ether,p-nitrosophenol, tert-butylphenols,1-oxyl-2,2,6,6-tetramethylpiperidin-4-ol or mixtures thereof. Use isfrequently made of phenothiazine in an amount of from 0.01 to 1% byweight.

Crude acrylic acid is then isolated from the absorption liquid ladenwith acrylic acid. When using a high-boiling liquid as absorptionliquid, the crude acrylic acid is usually separated off byrectification. The separation by rectification is advantageously carriedout under reduced pressure, e.g. from 0.04 to 0.1 bar, for example in apacked column or tray column. A polymerization inhibitor isadvantageously added at the top or in the upper region of therectification column. The crude acrylic acid can be taken off at thetop, but it is preferably taken off at a side offtake in the upperregion of the rectification column, with small amounts of impuritieshaving boiling points lower than that of acrylic acid, e.g. water andacetic acid, being taken off at the top of the column. The high-boilingliquid obtained after the crude acrylic acid has been separated off isadvantageously recirculated and reused for absorption. It is sometimesadvantageous for all or part of the residue which comprises mainly thehigh-boiling liquid to be treated thermally at above 180° C. before itis recirculated to the absorption column, so that ester-like oligomericacrylic acids present as impurities are dissociated and the resultingacrylic acid is distilled off together with the high-boiling liquid. Themaleic acid and maleic anhydride which are still present can be removedin a customary manner, e.g. by extraction with water, before thehigh-boiling liquid is reused.

If water is used as absorption liquid for absorbing the acrylic acidfrom the reaction gases, the crude acrylic acid is advantageouslyisolated from the initially obtained aqueous acrylic acid solution byextraction with an extractant and subsequent distillation of theextract. The extractant should have a high partition coefficient foracrylic acid and a low solubility in water and has to form an azeotropewith water. It is possible to use extractants which have boiling pointslower than that of acrylic acid, e.g. ethyl acetate, butyl acetate,ethyl acrylate, 2-butanone or mixtures thereof, or extractants havingboiling points higher than that of acrylic acid, e.g. t-butyl phosphate,isophorone or aromatic hydrocarbons. To carry out the extraction, theaqueous acrylic acid solution is advantageously passed through anextraction column in countercurrent to the chosen extractant.

Crude acrylic acid is then separated from the extract by distillation.The way in which the distillation is carried out depends on whether theextractant used has a boiling point higher or lower than that of acrylicacid. When using an extractant having a boiling point lower than that ofacrylic acid, the extract is, for example, fed to a solvent separationcolumn in which the extractant and residual amounts of water aredistilled off via the top. The bottom fraction from the solventseparation column is then fed to a low boiler column in which impuritieshaving boiling points lower than that of acrylic acid, e.g. acetic acid,are separated off at the top and crude acrylic acid is obtained asbottom fraction.

Instead of isolating crude acrylic acid from the reaction gases byabsorption in an absorption liquid, it is also possible to recover crudeacrylic acid by fractional condensation of the reaction gases, ifappropriate with subsequent purification by crystallization.

To carry out the fractional condensation, the reaction gases, whosetemperature has preferably been reduced to, for example, from 100 to180° C., by direct cooling using a cooling liquid, is advantageouslyintroduced into the lower region of the column containingseparation-active internals and is allowed to ascent within the column.A crude acrylic acid fraction can be taken off as intermediate-boilingfraction via an appropriately installed collection tray. Such a processis described, for example, in DE 19740253 or DE 19740252. A processpolymerization inhibitor, e.g. one of those mentioned above, isgenerally introduced into the column.

The crude acrylic acid fraction obtained in the fractional condensationcan be passed to a crystallization for further purification. Thecrystallization process is not subject to any restrictions. If acrystallization is employed, it is advantageously carried out as asuspension crystallization.

The crude acrylic acid obtained by the above method is treated with analdehyde scavenger. The aldehyde scavenger can be added directly into apipe by means of which the crude acrylic acid is passed to furtherwork-up or can be added in a residence vessel in which the crude acrylicacid is subjected to temporary storage before it is passed to furtherwork-up.

Suitable aldehyde scavengers include all compounds which convert thealdehydes present in the crude acrylic acid essentially quantitativelyinto compounds having a boiling point higher than that of acrylic acid.Nitrogen compounds having at least one primary amino group areparticularly suitable for this purpose. Examples which may be mentionedare aminoguanidine salts, hydrazine, alkylhydrazines and arylhydrazines,carboxylic acid hydrazides or aminophenols. Among these, particularpreference is given to aminoguanidine hydrogen carbonate. The aldehydescavenger is preferably used in an excess over the aldehyde present inthe crude acrylic acid, e.g. in an amount of from 1.5 to 2.5 mol permole of aldehyde. The reaction with the aldehyde scavenger can becarried out at from 15 to 50° C., preferably from 20 to 30° C. Areaction time of from 10 minutes to 72 hours, preferably from 2 to 50hours, is usually employed. If aminoguanidine hydrogen carbonate is usedas aldehyde scavenger, this firstly reacts too give off carbon dioxideand form aminoguanidine hydrogen acrylate. This reacts with the aldehydegroups of the aldehydes present to the corresponding iminoguanidinederivatives or their rearrangement products which are obtained as highboilers in the subsequent distillation. Treatment with the aldehydescavenger allows the residual aldehyde content of the crude acrylicacid, expressed as furfural, to be reduced to below 20 ppm, inparticular below 5 ppm, particularly preferably below 3 ppm.

Pure acrylic acid is then separated off from the crude acrylic acidwhich has been treated in this way by distillation. In this context,“separation by distillation” is meant in its broadest sense andencompasses both a simple distillation, i.e. a distillation in whichessentially no mass transfer occurs between condensate and vapor, and arectification in which part of the condensate is conveyed incountercurrent to the ascending vapor. It is a critical feature of thepresent invention that no fraction boiling at a lower temperature thanthe pure acrylic acid fraction is isolated, which leads to asimplification of the process and in general saves a distillation column(“low boiler column”). An advantageous method is to separate the treatedcrude acrylic acid thermally into vapor comprising acrylic acid and aresidue and to condense the vapor quantitatively to obtain pure acrylicacid.

The thermal separation is preferably carried out by simple distillation,i.e. essentially without reflux of condensate. Accordingly, use isadvantageously made of a distillation column without separation-activeinternals, i.e. a hollow column-like or tower-like structure which isgenerally made of stainless steel. To prevent droplets of crude acrylicacid from being entrained in the vapor comprising acrylic acid, thecolumn is advantageously provided with a droplet precipitator ofconventional construction, e.g. in the form of knitted wire packingwhich has a large internal surface area and can be made of, for example,chromium-nickel steels, aluminum, polypropylene, polytetrafluoroethyleneor the like, or in the form of a bed of random packing elements orordered packing, e.g. a stack of spaced, corrugated metal sheetsarranged parallel to the longitudinal axis of the column, having a smallheight of, for example, from 20 to 100 cm.

The temperature at the bottom is usually from about 65 to 130° C.,preferably from 70 to 100° C., and the column pressure is usually from50 to 120 mbar. Heating at the bottom of the column is provided by anexternal or internal circulation vaporizer, preferably a Robertvaporizer or a forced circulation decompression vaporizer. In vaporizersof the Robert type, a heating unit comprising vertical boiler tubes isaccommodated in a cylindrical vaporizer body. The crude acrylic acid ispresent in the interior of the boiler tubes. Circulation in the tubes iseffected by the rising bubbles of vapor. To recirculate the liquid whichhas been conveyed upward, one or more downcomers are installed in theheating unit.

Apart from the above-described heating at the bottom, the column ispreferably not actively heated; however, the wall of the column ispreferably insulated to avoid excessive heat loss by radiation. Theabsence of column heating (apart from the heating at the bottom) resultsin droplets of crude acrylic acid entrained in the vapor comprisingacrylic acid not being heated on their way through the gas space of thecolumn and their size not being reduced by evaporation of volatileconstituents. The entrained droplets, which retain their size or areenlarged by coagulation, can then readily be held back during passage ofthe vapor comprising acrylic acid through a droplet precipitator.

Apart from the wall of the column, the remaining parts of the plantwhich come into contact with the vapor comprising acrylic acid, inparticular the pipes through which the vapor comprising acrylic acid isconveyed before it is condensed, are provided with auxiliary heating toavoid undesirable premature condensation. Thus, the pipes can, forexample, be configured as double-wall tubes through whose annular spacebetween outer and inner wall a heating medium is circulated.Alternatively, it is possible to provide a tube through which a heatingmedium flows and which is in heat-conductive contact with the pipeconveying the vapor comprising acrylic acid and is, for example, woundhelically around the acrylic acid vapor pipe or runs parallel thereto.

In a preferred embodiment, the treated crude acrylic acid is separatedin a column provided with a circulation vaporizer into a first quantityof vapor comprising acrylic acid and a first residue, the first residueis separated in a film evaporator into a second quantity of vaporcomprising acrylic acid and a second residue, the first and secondquantities of vapor comprising acrylic acid are combined and condensedto give pure acrylic acid, and the second residue is discarded.

In this embodiment, the column is preferably operated so that the firstresidue amounts to at least 8% by weight, e.g. from 8 to 30% by weight,preferably from 10 to 25% by weight, of the crude acrylic acid fed tothe distillation column. This ensures that the residue has a handleable,not excessively high viscosity. In addition, it has been found that agreater degree of evaporation results in significantly greater foulingon the heat-exchange surfaces of the vaporizer employed for heating thebottom of the column, so that the plant has to be shut down and cleanedat shorter time intervals.

To recover the acrylic acid still present in the first residue, theresidue is fed to a film evaporator and a further quantity of vaporcomprising acrylic acid is obtained. Wiped film evaporators areparticularly useful as film evaporator. In these types, the liquid to beconcentrated by evaporation is distributed over a tube wall by means ofa rotating arrangement of wipers. Evaporators of the Sambay type areparticularly preferred. It has been found that film evaporators displaya reduced tendency to suffer from fouling because of their constructionand thus allow greater evaporation of the residue without interruptionfor cleaning than would be possible in the primary distillation column.The first residue is preferably concentrated in the film evaporator tofrom 35% by weight to 5% by weight, in particular from 10% by weight to20% by weight.

The second quantity of vapor is combined with the first quantity ofvapor, conveniently by recirculating the second quantity of vapor to thedistillation column. The second quantity of vapor is advantageouslyintroduced below a droplet precipitator provided in the distillationcolumn and the combined vapors are passed through the dropletprecipitator. This manner of operation has the advantage that only onecommon droplet precipitator is necessary to separate off entraineddroplets from the first and second quantities of vapor, which reducescapital costs and the need for cleaning. The residues obtained in thefilm evaporator, which correspond to, for example, from 0.5 to 5% byweight, generally from 1 to 2% by weight, of the total crude acrylicacid feed, are discarded.

In a preferred embodiment of the process of the present invention, thecrude acrylic acid which has been treated with the aldehyde scavenger isheated to from 40 to 110° C., preferably from 50 to 60° C., before beingintroduced into the distillation column. Heating is conveniently carriedout by indirect heat exchange, e.g. by means of a flow-through heatexchanger. Introducing preheated crude acrylic acid into thedistillation column has the advantage that a smaller quantity of heathas to be introduced at the bottom of the column via the vaporizerprovided for this purpose, which in turn leads to reduced fouling on itsheat-exchange surfaces.

In the thermal separation of the crude acrylic acid, it is advantageousto use a dissociation catalyst for ester-like oligomeric acrylic acid,in particular diacrylic acid. Suitable dissociation catalysts are, inparticular, acids such as alkylsulfonic and arylsulfonic acids, e.g.dodecylbenzenesulfonic acid or p-toluenesulfonic acid, or bases such assodium hydroxide or potassium carbonate. The dissociation catalyst isusually used in an amount of from 0.5 to 10 kg per metric ton of crudeacrylic acid. The dissociation catalyst can be added to the crudeacrylic acid feed or to the feed to the film evaporator.

The vapor comprising acrylic acid is then condensed to give pure acrylicacid. The condensation can be carried out by indirect heat exchange,e.g. in a heat exchanger, or preferably by direct heat exchange, e.g. ina gas cooler, by direct contact with a cooling medium. As coolingmedium, preference is given to using pure acrylic acid. The pure acrylicacid used as cooling medium preferably contains a storage polymerizationinhibitor, for example one of those mentioned above, in an amount of,for example, from 10 to 2000 ppm, preferably from 25 to 350 ppm.Hydroquinone monomethyl ether is particularly preferred for thispurpose. The condensation of the vapor comprising acrylic acid isusually carried out at 45° C. or below, generally at from 20 to 40° C.

Since the formation of polymers cannot be prevented completely even whenusing a polymerization inhibitor, fouling occurs on plant componentssuch as reactor walls, heat-exchange surfaces, on column trays and inlines and pumps, especially after prolonged running times. The plant orplant components and apparatuses therefore have to be cleaned from timeto time. For this purpose, the absorption column, the distillationcolumn, the film evaporator and/or other plant components which comeinto contact with the reaction gases, the high-boiling liquid laden withacrylic acid, the crude acrylic acid, the vapor comprising acrylic acid,the residues or the pure acrylic acid are treated periodically with anaqueous solution of a base.

Preference is given to emptying the plant component to be cleaned, ifappropriate rinsing it with water and treating it with the aqueoussolution of a base. According to EP-A 1033359, it is then advantageouslyrinsed with water. However, it is also possible to use an aqueoussolution of an inhibitor in place of water. The water used for rinsingbefore and after the base treatment is generally deionized water,condensate or mains water. As solution of a base, preference is given tousing a 5–25% strength by weight solution of potassium or sodiumhydroxide, with sodium hydroxide being preferred. Cleaning with thesolution of the base is usually carried out at from 20 to 100° C. forgenerally from 5 to 20 hours. The type and amount of deposits (fouling)naturally determine the cleaning conditions.

A final rinse with a 0.001–1% strength by weight aqueous solution of apolymerization inhibitor is particularly advantageous. As polymerizationinhibitors, preference is given to using phenolic compounds, e.g.hydroquinone, hydroquinone monomethyl ether, tert-butylphenols ornitrosophenols, N-oxyl compounds such as1-oxyl-2,2,6,6-tetramethylpiperidin-4-ol, methylene blue or mixturesthereof.

If a preliminary rinse with water is carried out, the solution obtainedis advantageously introduced into the work-up process for the reactiongases comprising acrylic acid from the gas-phase oxidation. Thewastewater from the base treatment is disposed of or worked up and/orwholly or partly reused. In this way, all or part of the aqueoussolution of the polymerization inhibitor can advantageously be used anumber of times. The final rinse with the inhibitor solution largelyprevents polymer formation during start-up of the plant and thus leadsto a longer running time of the plant.

The method of cleaning described above is not restricted to plants forpreparing acrylic acid, but can also be carried out advantageously inplants for preparing/purifying methacrylic acid and (meth)acrylicesters.

It is particularly advantageous to provide the columns and otherapparatus items with fixed rinsing lines via which the aqueous solutionof a base, water for preliminary or subsequent rinsing or the aqueousinhibitor solution can be introduced as necessary.

The pure acrylic acid obtained in this way can be used directly for thepreparation of absorbent resins, in particular superabsorbents(superabsorbent polymers or SAPs for short). A review of the preparationof SAPs based on acrylic acid may be found in F. L. Buchholtz and A. T.Graham (editors) in “Modern Superabsorbent Technology”, pp. 69–117, andthe references cited therein.

The preparation of SAPs based on acrylic acid is, as is known, carriedout by free-radical polymerization of aqueous monomer solutions whichcomprise essentially acrylic acid and/or its salts as polymerizablemonomers. The polymerization is preferably carried out as a solution orgel polymerization in a homogeneous aqueous phase or as a suspensionpolymerization in which the aqueous monomer solution forms the dispersephase. The hydrogels obtained in this way are subsequentlysurface-crosslinked.

The polymerization is advantageously carried out as a solutionpolymerization utilizing the Trommsdorff-Norrish effect (gelpolymerization). For this purpose, an aqueous, generally 10–70% strengthby weight and preferably 20–60% strength by weight, solution of amonomer mixture comprising acrylic acid is polymerized in the presenceof a substance which forms free radicals and optionally in the presenceof a suitable graft base.

In the polymerization process, the monomer mixture comprising acrylicacid is used in partially or fully neutralized form, i.e. the degree ofneutralization of all monomers bearing acid groups is in the range from20 to 100 mol %.

Apart from acrylic acid, the monomer mixture to be polymerized canfurther comprise other ethylenically unsaturated acids, e.g. methacrylicacid, vinylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, orneutral ethylenically unsaturated monomers, e.g. acrylamide,methacrylamide, N-vinyl amides such as N-vinylformamide,N-vinylacetamide, N-methylvinylacetamide, N-vinylpyrrolidone andN-vinylcaprolactam. The further monomers are usually used in an amountof less than 30% by weight, based on acrylic acid.

In general, crosslinker monomers are concomitantly used, usually in anamount of from 0.01 to 5% by weight, based on acrylic acid. Suitablecrosslinkers are N,N′-methylenebisacrylamide, polyethylene glycoldiacrylates and polyethylene glycol dimethacrylates, which are in eachcase derived from polyethylene glycols having a molecular weight of from106 to 8500, preferably from 400 to 2000, trimethylolpropanetri(meth)acrylate, ethylene glycol di(meth)acrylate, propylene glycoldi(meth)acrylate, butanediol di(meth)acrylate, hexanedioldi(meth)acrylate, triallylamine, dialkyldiallylammonium halides,divinylbenzene, diallyl phthalate, polyethylene glycol divinyl ethers ofpolyethylene glycols having a molecular weight of from 106 to 4000, andthe like.

Suitable graft bases can be of natural or synthetic origin. They includestarches, i.e. native starches selected from the group consisting ofmaize starch, potato starch, wheat starch, rice starch, tapioca starch,sorghum starch, manioc starch, pea starch or mixtures thereof, modifiedstarches, starch degradation products, e.g oxidatively, enzymatically orhydrolytically degraded starches, dextrins, e.g. dextrins obtained byroasting, and also lower oligosaccharides and polysaccharides, e.g.cyclodextrins having from 4 to 8 glucose units in the ring. Furtherpossible oligosaccharides and polysaccharides are cellulose, starchderivatives and cellulose derivativess. Also suitable are polyvinylalcohols, homopolymers and copolymers of N-vinylpyrrolidone, polyamines,polyamides, hydrophilic polyesters or polyalkylene oxides, in particularpolyethylene oxide and polypropylene oxide.

Polymerization reactors which can be used are the reactors customary forthe preparation of polymers; in the case of solution polymerization,preference is given to belt reactors, extruders and kneaders (cf.“Modern Superabsorbent Polymer Technology”, Section 3.2.3). The polymersare particularly preferably prepared by a continuous or batchwisekneading process.

Suitable initiators are, for example, peroxo compounds such as organicperoxides, organic hydroperoxides, hydrogen peroxide, persulfates,perborates, azo compounds and redox catalysts.

The polymers are generally obtained as hydrogels. Their moisture contentis generally in the range from 20 to 80% by weight. The hydrogelobtained in this way is then converted in a manner known per se into ahydrogel-forming powder and is subsequently surface-crosslinked. Forthis purpose, the hydrogel obtained in the polymerization is generallyfirstly comminuted by known methods. The rough comminution of thehydrogels is carried out by means of customary tearing and/or cuttingtools. The preferably neutralized or partially neutralized polymerobtained in this way is subsequently heated at, for example, from 80° C.to 250° C. This gives the polymers in the form of powder or granules,which may be subjected to further milling and screening procedures toadjust the particle size.

The subsequent surface crosslinking is carried out in a manner known perse on the resulting, dried, preferably milled and screened polymerparticles. Surface crosslinking is carried out using compounds whichhave at least two functional groups capable of reacting with thefunctional groups, preferably the carboxyl groups, of the polymer toproduce crosslinks (postcrosslinkers). For this purpose, thepostcrosslinkers are applied, preferably in the form of an aqueoussolution, to the surface of the polymer particles.

Suitable postcrosslinkers are, for example:

Ethylene glycol diglycidyl ether, bischlorohydrin ethers of polyalkyleneglycols, alkoxysilyl compounds, polyaziridines, diols and polyols andtheir esters with carboxylic acids or with carbonic acid, e.g. ethylenecarbonate or propylene carbonate, carbonic acid derivatives such asurea, thiourea, guanidine, dicyandiamide, 2-oxazolidinone and itsderivatives, bisoxazoline, polyoxazolines, diisocyanates andpolyisocyanates, di- and poly-N-methylol compounds such asmethylenebis(N-methylolmethacrylamide) or melamine-formaldehyde resins.

If necessary, acid catalysts such as p-toluenesulfonic acid, phosphoricacid, boric acid or ammonium dihydrogen phosphate can be added.

The crosslinker solution is preferably applied by spraying a solution ofthe crosslinker onto the polymer in conventional reaction mixers ormixing and drying units, for example Patterson-Kelly mixers, DRAISturbulent mixers, Lödige mixers, screw mixers, pan mixers, fluidized-bedmixers or a Schugi-Mix. After the crosslinker solution has been sprayedonto the polymer, a heat treatment step can follow.

A preferred embodiment of the process of the present invention isdescribed in more detail with the aid of FIG. 1 and the examples below.

FIG. 1 shows a plant suitable for producing pure acrylic acid. The hotreaction gas coming from the propane, propylene and/or acroleinoxidation, which comprises acrylic acid together with water vapor,acrolein, formaldehyde, formic acid, acetic acid, maleic anhydride andinert gases, is fed via line 1 into the absorber column 2. In theabsorber column 2, the temperature desired for the absorption is set bymeans of the liquid circuits 3 which are equipped with heat exchangers4. The high-boiling liquid is introduced at the top of the absorbercolumn 2 via line 5 and scrubs the acrylic acid from the reaction gasesin countercurrent. The constituents of the reaction gas which have notbeen absorbed leave the absorber column 2 via line 6. The liquid flowingout from the absorber column 2 is conveyed via line 7 through the heatexchanger 8 where it is heated to the top of the desorber column 9.Stripping gas is fed into the desorber column 9 at the bottom via line10. The stripping gas travels in countercurrent to the downflowingliquid laden with acrylic acid and substantially frees it of volatileimpurities. The gas leaving the top of the desorber is conveyed via line11, if desired also via line 12 or 13, to the absorber column 2.

The liquid flowing out from the bottom of the desorber column 9 isconveyed via line 14 to the distillation column 15. Crude acrylic acidis taken off in the upper region of the distillation column 15 via line16. Remaining low boilers are removed at the top of the distillationcolumn 15 via line 28. The high-boiling liquid which has been freed ofacrylic acid can, if appropriate after work-up, be recirculated aftercooling in the heat exchanger 18 to the top of the absorber column 2.

The crude acrylic acid taken off at the top of the distillation column15 is fed via line 16 to the reaction vessel 19 into which an aldehydescavenger is metered via line 20. After an appropriate reaction time,the crude acrylic acid is conveyed via line 21 to the column 22 which isfree of internals and provided only with a droplet precipitator 23. In apreferred embodiment, a heat exchanger is provided in line 21 to preheatthe crude acrylic acid. The vapor comprising acrylic acid which leavesthe column 22 is taken off via line 28 and can be condensed to give pureacrylic acid. The high-boiling residue at the bottom of the column 22 isfed via line 24 to an evaporator 25. The vapor obtained is conveyed vialine 26 back to the column 22 and fed into the latter below the dropletprecipitator 23. The concentrated residue from the evaporator 25 istaken off via line 27 and disposed of.

EXAMPLE 1

In an apparatus as shown in FIG. 1, the hot reaction gases (230° C.)from the oxidation of propene, which had been carried out in a knownmanner by means of atmospheric oxygen in two stages in the gas phaseover multimetal oxide catalysts, were fed via line 1 to the absorbercolumn 2. The column was equipped with 35 bubble cap trays and twoexternal heat exchangers. At the top of the column, 0.7 kg of a solventmixture comprising 58.8% by weight of diphenyl ether, 21.2% by weight ofbiphenyl and 20% by weight of dimethyl o-phthalate and containing 0.1%by weight of phenothiazine and having a temperature of 45° C. was fed in(line 5) for every 1000 l of reaction gases. The heat exchangers wereoperated so that the gas temperature after the 2^(nd) heat exchanger wasabout 60° C. The liquid comprising acrylic acid which flowed out at thebottom of the column was brought to a temperature of 105° C. by means ofthe heat exchanger 8 and fed to the desorber column 9 (25 bubble captrays). 200 l of air heated to 90° C. per kg of liquid from the absorbercolumn were blown at the bottom. The stripping gases laden with lowboilers were fed back to the absorption column. The liquid flowing outfrom the column 9 was fed to the distillation column 15 (43 dual-flowtrays, external circulation vaporizer, temperature at the bottom=175°C., pressure at the top=100 mbar) on the 5^(th) tray. Liquid crudeacrylic acid was discharged via a side offtake (35^(th) tray); thiscomprised, inter alia, the following components:

Acrylic acid 99.3% by weight Diacrylic acid 0.2% by weight Acetic acid0.15% by weight Propionic acid 0.04% by weight Furfurals 0.4% by weightBenzaldehyde 0.01% by weight Water 0.1% by weight Phenothiazine 0.05% byweight

The low boilers discharged at the top of the column, mainly water andacetic acid, contained 2% by weight of acrylic acid, and the solventmixture obtained at the bottom of the column contained about 1% byweight of acrylic acid and was recirculated to the absorption column.The crude acrylic acid was treated in a stirred vessel (19) with thedouble molar amount (based on furfural and benzaldehyde) ofaminoguanidine hydrogen carbonate at 23° C. for 10 hours. The mixturewas subsequently heated to 50° C., admixed with 0.3% by weight ofdodecylbenzenesulfonic acid and fed to the bottom of the distillationcolumn 22. The column was equipped with a spray precipitator (23) and acirculation vaporizer. The temperature at the bottom was 85° C., and thepressure was 90 mbar. The gaseous acrylic acid discharged was condensedby quenching with liquid acrylic acid and was stabilized by addition of200 ppm of hydroquinone monomethyl ether (MEHQ). For this purpose, anabout 1.5% strength by weight solution of MEHQ in acrylic acid wascontinuously metered in in the required amount.

The acrylic acid remaining in the bottom product (15% by weight of thefeed) was mostly distilled off in a Sambay evaporator (25) (75° C., 70mbar), and the vapor was introduced into the column (22) below the sprayprecipitator (23).

The condensed pure acrylic acid had the following composition:

Acrylic acid 99.7% by weight Diacrylic acid 0.01% by weight Acetic acid0.14% by weight Propionic acid 0.04% by weight Aldehydes <5 ppmPhenothiazine <1 ppm Water 0.11% by weight

The bottoms from the Sambay distillation (about 2% by weight of the feedto column 22) contained about 20% by weight of acrylic acid and weredisposed of. About 99% by weight of the acrylic acid present in thecrude acrylic acid were recovered. The distillation unit could beoperated without problems for more than 30 days.

A 40% strength aqueous solution of the pure acrylic acid was preparedand neutralized with sodium hydroxide. The solution was admixed with0.50% by weight, based on acrylic acid, of crosslinker (diacrylate of apolyethylene glycol having a mean molecular weight of 400). Asinitiator, the following system was employed:

-   0.005% by weight of hydrogen peroxide and-   0.006% by weight of ascorbic acid and-   0.28% by weight of sodium peroxodisulfate,    based on acrylic acid.

The individual components of this reaction solution (dilute aqueoussolutions of hydrogen peroxide, ascorbic acid, sodium peroxodisulfateand the monomer/crosslinker solution) were metered separately into akneader as reactor and mixed while running into the reactor with thepolymerization starting quickly even during mixing.

600 kg/h of reaction solution were introduced and the gel produced bypolymerization in the kneader was discharged continuously. Thetemperature of the cooling water in the reactor jacket was regulated to90° C. During the polymerization, 14 m³/h of nitrogen were passedthrough the kneader as inert gas. The reaction volume was 300 l.

The gel which had been discharged was dried, milled and screened toproduce a particle size fraction of 100–800 μm.

1. A process for preparing water-absorbent resins, which comprises: a)obtaining crude acrylic acid by either a1) absorbing acrylic acid fromthe reaction gases obtained from the catalytic gas-phase oxidation ofpropane, propylene and/or acrolein in an absorption liquid and isolatingcrude acrylic acid from the absorption liquid laden with acrylic acid,or a2) separating a crude acrylic acid fraction from said reaction gasesby fractional condensation and, optionally, purifying the condensedfraction of acrylic acid by crystallization, b) treating the crudeacrylic acid obtained from a1) or a2) with an aldehyde scavenger, c)separating pure acrylic acid from the treated crude acrylic acid bydistillation, and d) polymerizing the pure acrylic acid, optionallyafter partial neutralization, alone or in admixture with otherethylenically unsaturated monomers, under free-radical polymerizationconditions, wherein the isolation of the pure acrylic acid bydistillation is conducted without reflux of condensate by thermallyseparating the treated crude acrylic acid into vapor comprising acrylicacid and a residue and quantitatively condensing the vapor to obtainpure acrylic acid, no fraction boiling lower than the pure acrylic acidfraction being isolated.
 2. The process as claimed in claim 1, whereinthe absorption liquid laden with acrylic acid is stripped by means of astripping gas to remove volatile impurities.
 3. The process as claimedin claim 1, wherein the aldehyde scavenger is aminoguanidine hydrogencarbonate.
 4. The process as claimed in claim 1, wherein the treatedcrude acrylic acid is separated in a column provided with a circulationvaporizer into a first quantity of vapor comprising acrylic acid and afirst residue, the first residue is separated in a film evaporator intoa second quantity of vapor comprising acrylic acid and a second residue,the first and second quantities of vapor comprising acrylic acid arecombined and condensed to give pure acrylic acid, and the second residueis discarded.
 5. A process as claimed in claim 1, wherein the absorptionliquid contains a polymerization inhibitor.
 6. A process as claimed inclaim 1, wherein the absorption liquid is diphenyl ether, biphenyl,dimethyl o-phthalate or a mixture thereof.
 7. The process as claimed inclaim 1, wherein the aldehyde scavenger, is aminoguanidine hydrogencarbonate.
 8. The process as claimed in claim 2, wherein the treatedcrude acrylic acid is separated in a column provided with a circulationvaporizer into a first quantity of vapor comprising acrylic acid and afirst residue, the first residue is separated in a film evaporator intoa second quantity of vapor comprising acrylic acid and a second residue,the first and second quantities of vapor comprising acrylic acid arecombined and condensed to give pure acrylic acid, and the second residueis discarded.
 9. The process as claimed in claim 3, wherein the treatedcrude acrylic acid is separated in a column provided with a circulationvaporizer into a first quantity of vapor comprising acrylic acid and afirst residue, the first residue is separated in a film evaporator intoa second quantity of vapor comprising acrylic acid and a second residue,the first and second quantities of vapor comprising acrylic acid arecombined and condensed to give pure acrylic acid, and the second residueis discarded.
 10. The process as claimed in claim 2, wherein theabsorption liquid contains a polymerization inhibitor.
 11. The processas claimed in claim 3, wherein the absorption liquid contains apolymerization inhibitor.
 12. The process as claimed in claim 4, whereinthe absorption liquid contains a polymerization inhibitor.
 13. Theprocess as claimed in claim 2, wherein the absorption liquid, isdiphenyl ether, biphenyl, dimethyl o-phthalate or a mixture thereof. 14.The process as claimed in claim 3, wherein the absorption liquid, isdiphenyl ether, biphenyl, dimethyl o-phthalate or a mixture thereof. 15.The process as claimed in claim 2, wherein the absorption liquid, isdiphenyl ether, biphenyl, dimethyl o-phthalate or a mixture thereof. 16.The process as claimed in claim 2, wherein the absorption liquid, isdiphenyl ether, biphenyl, dimethyl o-phthalate or a mixture thereof. 17.The process as claimed in claim 1, wherein acrylic acid is separatedfrom the absorbing liquid by a stripping gas in a column.